Tradition trumped teachers' disapproval as hundreds of students made a splash seeing out the school year.

Nearly 250 year 12 pupils from Wellington Girls' College leapt into the Frank Kitts Park lagoon to celebrate their last day in uniform this afternoon.

Students are permitted to wear mufti to school during year 13.

Ata Bridge-Comer, 17, said the plunge was a school tradition.

"Everyone knows that on your last day it's tradition to come down here.

"Everyone looks forward to it from year nine."

Despite the best efforts of organiser Briana Wu to hold back the screaming throng, one girl threw herself into the water about 20 minutes before the scheduled 1pm start - prompting hundreds of her peers to follow suit.

"They’re like lemmings," observed one bemused parent.

Miss Wu, 16, said the annual event was organised in the face of teachers' disapproval.

"It's been going on for ages, but it’s a little bit rogue.

"The teachers don't want anything to do with it.

"I wasn't allowed to say anything officially - it was more word-of-mouth."

Grace Surridge, 17, said teachers knew to turn a blind eye.

"They don't agree with it, but because it's tradition, they have to let it happen."

All students were expected to take part, she said.

"Everyone has to do it, no excuses."

Pointing at a group of her peers, conspicuously dry on the other side of the lagoon, she asked "Why do you reckon they didn't jump?"